1. Field of the Invention
The present invention relates generally to a fuel cell, and more particularly to a fuel cell using a polymer electrolyte membrane or the like.
2. Description of the Related Art
A fuel cell is an electrochemical energy conversion device. Fuel cells use an electrolyte membrane to catalytically react an input fuel, such as hydrogen, with an oxidant, such as oxygen, to produce an electrical current. The electrolyte membrane is sandwiched between two electrodes (an anode and a cathode). A catalyst on the anode promotes the oxidation of hydrogen molecules into hydrogen ions (H+) and electrons. The hydrogen ions migrate through the electrolyte membrane to the cathode, where a cathode catalyst causes the combination of the hydrogen ions, electrons and oxygen, producing water. The electrons go through an external circuit that serves as an electric load while the ions move through the electrolyte toward the oppositely charged electrode. At the second electrode, the ions combine to create by-products of the energy conversion process, the byproducts being primarily water and heat. The flow of electrons through an external circuit produces electric current.
There are several types of fuel cells employing different types of electrolyte membranes, including: a phosphoric acid fuel cell, a molten carbonate fuel cell, a solid oxide fuel cell, and a polymer electrolyte membrane fuel cell, also referred to as a proton exchange membrane fuel cell.
The type of fuel cell that involves a polymer electrolyte membrane is hereinafter referred to as a PEM fuel cell. Developments in PEM fuel cell technology have produced fuel cells suitable for applications where the fuel cell will remain dormant for long periods of time before producing energy through electrochemical reaction. PEM fuel cells may include two very small storage tanks to hold the fuel and oxidant gases, such as hydrogen and oxygen, while the fuel cell is dormant. This type of storage tank is sometimes referred to as nanotechnology storage because of its small size. The reaction is initiated after the period of dormancy by the act of fracturing, puncturing, rupturing, or otherwise releasing the gases from the storage tanks to the PEM for the electrochemical reaction.
Work on PEM type fuel cells has produced fuel cells in the size range of 0.2 millimeters in thickness and capable of running for over 60,000 hours at 80 degrees Celsius. These PEM fuel cells are capable of producing better than 400 mA (milliamperes) of current per square centimeter, at 0.7 volts, in some applications, depending on whether air or oxygen is used on the cathode. The fuel cells may be stacked to deliver higher voltages. However, despite the advancements made in miniaturization of fuel cells, a fuel cell stacking arrangement is not feasible for some applications due to dimensional limitations of some environments where the fuel cells may be used.
For applications where fuel cells of the type described are to replace lithium reserve battery units, known to have a more limited shelf life, the cells may have to be accommodated within a physical location that affords a limited height to width ratio. In such applications, dimensions may be limited to a range of as little as ½ inch high and 1½ inch diameter. As stacked fuel cell assemblies usually exceed such dimensional limits, alternative fuel cell designs are necessary.
Required fuel cell performance under certain operational conditions is determined both theoretically and experimentally. When determining required performance of a fuel cell, different operating characteristics must be evaluated because the fuel cell will operate under a variety of abnormal conditions. For example, the fuel cell will provide energy below the normal Polymer Electrolyte Membrane fuel cell operation temperature of around 80 degrees Celsius. Fuel cells are also capable of running on pure oxygen or air, at pressures higher than atmospheric, and without hydration.
According to DuPont, Inc., the manufacturer of Nafion®, one of several possible membrane materials that may be used in the fuel cell, operating characteristics such as higher pressure and pure oxygen as the oxidant gas will improve performance of the fuel cell from the performance under normal conditions. However, though the fuel cell will operate without hydration, lack of hydration reduces fuel cell performance and can offset improved performance that results from other positive changes in operating conditions.
Available literature indicates that this increase in performance under certain conditions is due to a higher Gibbs free energy value. When one or more of the potential driving forces behind a chemical reaction is favorable and other factors are not, the Gibbs free energy value (G) reflects the balance between these forces. Gibbs free energy is measured by the relationship between system enthalpy and system entropy. The change in Gibbs free energy that occurs during a reaction is equal to the product of the change in temperature and the change in entropy of the system subtracted from the change in enthalpy of the system.
Performance curves can be generated to predict fuel cell voltage and current values of stacked membrane assemblies and alternative fuel cell configurations. In FIG. 4, a collection of performance curves has been generated to show the performance of a fuel cell under various conditions as indicated in the caption under the graph. The four performance curves grouped together on the higher portion of the chart in FIG. 4 show the expected performance of a hydrated fuel cell at various conditions. The conditions indicated are two different operating temperatures, 22 degrees C. and 80 degrees C. and two different pressures, 14.7 psi and 500 psi. The two performance curves toward the bottom of the chart in FIG. 4 show the expected performance without hydration, where one is for a fuel cell having the size of a D-size battery and the other curve is for a fuel cell according to the present invention, which is indicated as MOFA for Multi-Option Fuse for Artillery. The curve toward the top of each series demonstrates the performance of the Polymer Electrolyte Membrane (PEMERY™) battery curve, while the curve labeled “D” Size indicates where the performance of a typical D-sized PEMERY™ style battery would fall on the chart.
Another limitation presented by the environments in which polymer electrolyte fuel cells may be used is the ways in which the electrochemical reaction may be initiated after the long period of dormancy. The inventor has developed piston-type activators that can be used to initiate reaction in a fuel cell, but such activators are generally not easily adapted for use in all applications.